Mechanics of Interfacial Composite Materials
Experiments have shown that if a fluid/fluid interface is covered with particles that are sterically jammed, the interface can exist in stable non-spherical shapes. The jammed particles thus allow the interface to rheologically behave in some ways like a solid. In this article, the authors examine the effects of small homogeneous and inhomogeneous stresses on this granular medium, or "armored bubble". They characterize the armored interface as an interfacial composite material (ICM) because the interfacially trapped particles retain their individual characteristics.
Date: May 24, 2006
Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA.
Langmuir 2006, Vol. 22, No. 24, 10204-10208. 
Armored interfaces are fluid/fluid interfaces covered with rigid particles typically larger than a micron in diameter. Several recent experiments have shown that armored interfaces are capable of supporting bubbles or droplets with various nonspherical geometries (see Figure 1). It can be shown that armored interfaces have the attributes of composite materials, because the system exhibits collective mechanical properties distinct from the properties of the constituent particles and fluid/fluid interface. Here the authors use systems of nonspherical armored bubbles and they find that the ICMs respond plastically to small inhomogeneous stresses and rigidly to minute homogeneous stresses.The fluid/fluid interface exerts a capillary force on adsorbed particles, which causes them to sterically jam. One simple way to create a nonspherical armored bubble is to fuse two spherical armored bubbles together. The particles constrain the surface area of the resulting object while the volume of the bubble must also be conserved, so the result is that the bubble maintains an ellipsoidal shape. Thus, a defining property of particle-covered bubbles is the conservation of both surface area and volume. A long, spherocylindrical bubble can be created in this manner which does not undergo a Rayleigh-Plateau capillary instability, demonstrating that the ICM is behaving like a solid.
One can probe the solidlike properties of armored bubbles by noting that because of their geometry, they should be quite resistant to homogeneous stresses, such as those experienced during a simple shear flow. As expected, when this experiment was performed, the bubbles tumbled like solid objects. The system's response to inhomogeneous stresses can also be explored by subjecting an ellipsoidal armored bubble to uniaxial compression along its major axis, as shown in Figure 2(a-d). The ICM undergoes plastic rearrangement and eventually extends a major axis orthogonal to the original. When the side plates that produced the compression are removed, the ellipsoid is stable in this new configuration. Thus, the bubble undergoes extensive plastic deformation when subject to uniaxial compression and no colloidal particles leave the monolayer during the process.
The inset diagram and graph at the bottom of Figure 2 indicates that shearing a hexagonally close-packed (HCP) monolayer is impossible for constant surface area. The microscopic strain is about 0.15 when the particles are square-packed but work must be done to increase the surface area. Just past this point, the work put in to create the excess surface area is regained elastically as the particles can assume a HCP configuration again. Once this yield strain is reached, the particles flow continuously on the interface if the applied stress is constant. Localized events thus seem to determine the microscopic response of the system to large-scale forces.
Connection to soft matter
This paper presents an example of how a system traditionally characterized as one phase in soft matter may, under the right conditions, exhibit properties typically not associated with that phase at all. In this case, if a fluid-fluid interface is studded with small jammed particles, it behaves as a granular solid that exhibits a plastic response to inhomogeneous stresses and an elastic response to small homogeneous stresses. This system displays unusual rheological properties, including an unusually high relative yield stress. Another fascinating property is that these bubbles conserve both volume and surface area when they coalesce, unlike bubbles that are not armored, which generally minimize surface area due to surface tension.
One interesting implication of the fact that armored interfaces behave like composite materials is that additional degrees of freedom can be obtained by varying the size and type of particles used prior to coalescence events. Since the mechanical properties of the ICM are related to the particle size, having sections of an armored interface with varying particle sizes should yield novel mechanical behavior. These systems could also be investigated as model two-dimensional crystalline surfaces. They have application potential for bubble and droplet transport and delivery processes.